While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are U.S. Pat. Nos. 3,172,862 and 3,173,050; J. Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and U.S. Pat. No. 3,710,167. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
Herein, the term “EL element” encompasses the layers between the anode and cathode. Recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Reducing the thickness lowers the resistance of the organic layer and has enabled devices that operate using much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes; therefore, it is referred to as the hole-transporting layer (HTL). The other organic layer is specifically chosen to transport electrons, and is referred to as the electron-transporting layer (ETL). The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pairs which results in light emission.
Additional layers have been proposed to further improve device performance, e.g., as described in U.S. Pat. No. 4,769,292. This patent discloses an organic light-emitting layer (LEL) between the HTL and ETL, and the concept of a hole-injecting layer (HIL) located between the anode and the HTL.
Materials comprising porphyrinic compounds have been disclosed by Tang in U.S. Pat. No. 4,356,429 for use in the HTL. Further improvements in device performance are taught in U.S. Pat. Nos. 4,539,507; 4,720,432; and 5,061,569 where the hole-transporting layer utilizes an aromatic tertiary amine.
Since these early inventions, further improvements in hole-transporting and other device materials have resulted in improved device performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569; 5,409,783; 5,554,450; 5,593,788; 5,683,823; 5,908,581; 5,928,802; 6,020,078; and 6,208,077, amongst others. EP 891,121 and EP 1,029,909 suggest the use of biphenylene and phenylene diamine derivatives to improve hole injecting and/or transporting and JP 11-273830 suggests the use of naphthyldiamine derivatives in EL elements generally.
Notwithstanding these developments, there are continuing needs for organic EL device components that will provide a desired high luminance.